I first bought the 4 foot version as you see in the pictures.  I was told that is best length to work with for most work.
4 feet was just too hard to work with when blowing out the engine compartment on combine and spots on tractors.  I ended up buying a shorter version - 2 footer and use that the most now.  With the amount of airflow I found I do not have to have nozzle right on spot blowing to clean off most dirt.  I put another holder on the trailer so have both with me.  4 footer is nice for blowing off tillage equipment for reaching further.
Since they are not cheap, not sure what I would recommend to start with. I think the shorter is easier to work with .  You can always make a cheap long blow bun with a ball valve to get to hard to reach places.